1. Field of the Invention
The present invention relates to an image reading apparatus using a lineimage sensor in a facsimile apparatus or the like.
2. Related Background Art
A conventional apparatus of this type uses a CCD type line image sensor. Main scanning of a read original is performed by this sensor, and subscanning is performed by relatively moving the sensor and the original in a direction perpendicular to the line direction of the sensor.
The above sensor has the following structure.
The sensor comprises photodiodes arranged in an array, storage electrodes, arranged in a one-to-one correspondence with the photodiodes, for storing charges generated by electromotive forces of the photodiodes, and a charge transfer element (CCD) for receiving the charges stored in the storage electrodes and sequentially shifting the charges, thereby outputting the charges.
In a conventional read apparatus, a read operation must be completed within a short storage time so as to read the original at high speed. For this purpose, a bright light source such as a fluorescent lamp is required.
An inverter is required to keep the fluorescent lamp on with an RF power. The fluorescent lamp may become an unnecessary radiation source, a large amount of heat is generated to result in a short service life, and the resultant apparatus is bulky and has a high cost.
When an LED array is used as a light source, the above problem is solved, but the amount of light generated by the LED array is small. It is therefore difficult to read an original at high speed.
When the CCD sensitivity (S/N ratio) is increased, a high-speed read operation can be realized even with an LED array light source.
The CCD sensitivity (S/N ratio) is almost proportional to the area of the photodiode opening surface per pixel. When the magnifications in the main scanning and subscanning directions are increased at the same rate so as to increase the total sensitivity, the CCD chip size is increased at the same rate as the magnification of pixels in the main scanning direction (longitudinal direction), resulting in high cost.
When the magnification of pixels is increased in only the subscanning direction, the sensitivity is increased while requiring almost no increase in CCD chip size. In this case, however, the resolution in the subscanning direction is undesirably decreased.